You can go in the admin web interface and disable the auto-provisioning features in the OBi. I really don't think Obihai will remove the Google Voice functionality unless / until XMPP really stops working, but disabling the auto provisioning functionality will prevent you from configuring your OBi on the web (and you have to use the "expert" interface locally).
Nonetheless, I don't think Obihai will also prevent you from downgrading your firmware so even in that case you can probably reflash an old version (if you are talking about them dropping XMPP support)...